In the West, web users have known for years that advertisers drop "cookies" onto their desktops (via their web browsers), and that these little pieces of code tell advertisers what they're looking at.

In China, however, the state-run TV channel China Central Television just discovered this fact. It aired an investigative, undercover hidden-camera story on the web ad business as a purveyor of secret tracking information on innocent Chinese web users.

Cookies help advertisers target people with ads. If you browse a web site for tennis rackets, you might start seeing ads for shoes on subsequent pages. Cookies don't, however, identify individual web users. They simply aggregate them into blocks of targetable audiences.

Using hidden cameras, a CCTV reporter apparently posing as a prospective client had conversations with employees at five local digital-ad agencies. Agency employees told the reporter they use cookies to access web users' personal information, including gender, age, marital status, education, salary and email addresses, to more accurately target consumers with online advertising. The story featured footage from what appeared to be the offices of agencies Yoyi and Avazu. The agencies were not given a chance to respond to the allegations.